1. Field of the Inventions
The present inventions relate to cooling systems for small boats, including water pipe attachment structures for small boats in which cooling water from outside the boat is used to cool the engine.
2. Description of the Related Art
Certain typical water-cooled marine engines include a water jacket that is configured to cool the engine, including the cylinder block and cylinder head. The cooling systems of some of these marine engines are known as “open-loop” cooling systems, which use water from the body of water in which the associated boat is operating for cooling the engine.
For example, Japanese Patent Document JP-A-2002-242673 discloses such a boat. In such small boats, cooling water is supplied to the engine via cooling water piping or “plumbing”. An example of attachment structure of the cooling water piping is shown in FIG. 10.
As shown in FIG. 10, a cooling water attachment structure 40 is provided in the vicinity of a water cooling jacket part 43 formed in a cylinder body 42 of an engine 41. The structure 40 includes three pieces of cooling water piping 44a, 44b and 44c, a cooling water piping attachment part 45 attached to the main body of the engine 41, and a pressure regulating valve 46 installed in the cooling water piping attachment part 45. A cooling water inlet/outlet port 47 is formed between the water cooling jacket part 43 and the cooling water piping attachment part 45. A cooling water passage 48 for communicating the cooling water piping 44a, 44b and 44c with the cooling water inlet/outlet port 47 is formed in the cooling water piping attachment part 45.
Cylindrical connection parts 49a, 49b and 49c for respectively fixing the cooling water piping 44a, 44b and 44c are formed on a surface of the cooling water piping attachment part 45. The cooling water piping 44a, 44b and 44c, respectively, surround the outer periphery of the connection parts 49a, 49b and 49c. 
When the engine 41 is in operation, cooling water is supplied primarily from the cooling water piping 44a via the cooling water passage 48 and the cooling water inlet/outlet port 47 into the water cooling jacket part 43 to cool the engine 41. Some of the cooling water is supplied into the water cooling jacket part 43 from the cooling water piping 44b. 
When the engine 41 is stopped, the cooling water in the water cooling jacket part 43 is discharged out of the boat through the cooling water inlet/outlet port 47, the cooling water passage 48 and the cooling water piping 44b. When the pressure in the water cooling jacket part 43 is increased, the pressure regulating valve 46 is actuated to discharge the cooling water in the water cooling jacket part 43 to the outside through the cooling water piping 44c. 